The use of ion conducting membranes or electrochemical transfer walls to transfer ions between two separate compartments of electrochemical cells has been known for a long time. The concept is used commercially in a variety of applications where processes based on one or more electrochemical reactions are improved by separating the reaction that occurs at the anode from the reaction that occurs at the cathode. Classically this separation is achieved using materials that permit cations or anions to be transferred preferentially over the solvent and other chemical species through a micro/nano-porous network (porous ion conducting membranes, for example polymer ion conducting membranes) or through conduction pathways created by the crystal structure of the specific material (dense ion conducting membrane, for example ion conducting ceramic or ion conducting glass membranes). In both cases, migration due to an external electric field causes ions to transfer through the separating material without the ion or the material changing its electronic state.
Conversely, U.S. Pat. No. 8,449,747 B2 describes the use of an electrochemical transfer wall to separate two electrochemical reactions occurring in separate compartments, the wall being made of a material that permits the intercalation of ions into it on the first side of the wall and de-intercalation out of it on the second side of the wall. U.S. Pat. No. 8,449,747 B2 and U.S. Publication No. 2013/0126354 A1 teach that the difference between an ion conducting membrane and electrochemical transfer wall is the diffusion of ions through the electrochemical transfer wall is accompanied by an electron transfer from the wall and then a subsequent electron transfer to the wall. This electron transfer causes the material to undergo a change in electronic structure during the intercalation, the diffusion, and the de-intercalation of the ion.
Under an electric field, porous ion conducting membranes, such as polymer ion conducting membranes, permit the ions to transfer from a first medium to a second medium through micro/nano-porous networks. Depending on presence of counter ions in the pores these membranes can preferentially conduct either positive or negative ions. These membranes are used commercially for a variety of electrochemical processes due to their relatively high conductivity, ease of manufacture and desirable physical properties. Currently, there are many limitations to porous ion conducting membranes or areas which industry requires improvement. Some non-limiting examples of these limitations will now be given.
First they permit the conduction of all similarity charged species, for example a cationic polymer membrane will transfer any positively charge species from a first solution to a second solution as long as it is smaller than the smallest pore diameter, otherwise it will clog the pores and inhibit the transfer of other ions. This is also true for the conduction of anions through anionic polymer membranes. The lack of selectivity in porous ion conducting membranes reduces the benefit of using a membrane to separate the reactions occurring at the two electrodes. In either case when the porous membranes are not under an applied electric field, the separation afforded by the electric field will be lost, and the concentration gradients generated by the electrical field will cause the ions to diffuse back through the membrane. These types of membranes can also foul easily due to ions or other objects present in the electrolyte that are larger than the smallest pore, but smaller than the larger pores, migrating into and become lodged in the membrane, inhibiting any further ion transfer through that pathway. In summary, the inherent properties of porous ion conducting membranes, inhibits them from truly keeping the medium on one side of the membrane from interacting with the medium on the other side of the membrane.
Under an electric field, dense ionic conducting membrane, such as ceramic or glass ionic conducting membranes, permit ions to transfer from a first medium to a second medium through conduction pathways present in their crystal structure or grain boundaries. This permits dense membranes without micro/nano-pores to be used to separate the reactions occurring at the anode and cathode. The transfer of ions in dense ion conducting membranes is therefore selective to ions that can be inserted into the crystal structure of the material without causing an irreversible change to the crystal structure. This selectivity addresses many issues found for porous ion conducting membranes including inhibiting the first medium from contaminating or being contaminated by the second medium, the membranes do not foul due to pore blockage, and they do not permit any back diffusion to occur. Also because the diffusion is occurring within the crystal structure and not from micro/nano-pores the only communication between the two mediums is the ion which can be transferred by the crystal-structure or grain boundary of the material making up of the membrane. Thus, dissimilar mediums can be used that are specifically chosen for the two electrochemical reactions that are occurring at the electrodes. One limitation with dense ion conducting membranes is there are only a limited number of known materials which conduct a limited number of ions at conditions that are amendable to practical applications.
Under an electric field, an electrochemical transfer wall or junction, U.S. Pat. No. 8,449,747 B2, permits ions to transfer from a first solution to a second solution by first intercalating the ion at the interface between the wall and the first solution and then migration of the ion through the wall, followed by the de-intercalation of the ion at the interface of the wall and the second solution. Intercalation and de-intercalation as used herein, refer to a process involving both ion and electron transfer at the interface. Thus, the electronic state of the wall changes as the ion is transferred from a first solution to a second solution. Like ionic conducting ceramic membranes, the electrochemical transfer wall is a non-porous dense layer that is not limited by the issues that are present with porous ion conducting membranes. But unlike, the dense ion conducting membranes, there are numerous materials that are known to undergo intercalation reactions. This increases the number of electrochemical systems that can benefit from separating the reactions occurring at the anode or cathode.
The limitations with electrochemical transfer walls are the requirement of having an electron transfer accompany the ion transfer at the interface of the wall and first solution, the requirement of having a second electron transfer at the interface of the wall and second solution, and the limitation this forces on the current density for which the ion can be transferred across the wall (U.S. Pat. No. 8,449,747 B2, <5 mA/cm2) or higher current densities through more elaborate cell design and setup (U.S. Publication No. 2013/0126354 A1). These limitations narrow the field of use for electrochemical transfer walls to applications that can tolerate low current densities and complicated electron transfer.
There is need for method and device that will permit the transfer of ions between two compartments of an electrochemical system at current densities found with porous ion conducting membranes, but with the benefits afforded by dense ion conducting membranes and with the ability to transport a diverse and large number of ions afforded by electrochemical transfer walls. There are numerous applications where this type of device could find use, such recycling industrial waste, recycling mining waste, removing metals from petroleum residuals, synthesizing different chemicals, removing ions from solution such as desalination.